Brines are widely used in well completion as a kill fluid, completion fluid, packer fluid or workover fluid, etc. Generally, a zinc bromide (ZnBr2) brine will be used when a brine is required to have a density of about 14.0 ppg or above, or even lower if low True Crystallization Temperature (TCT) fluid is required. Depending on the economic concern and the requirements of their properties, the zinc bromide fluids applied include CaBr2/ZnBr2 two-salt and CaCl2/CaBr2/ZnBr2 three-salt brines.
During completion of the well, however, such brines may become lost in the reservoir and remain in the formation for a long period of time. If a reservoir contains hydrogen sulfide (H2S) gas, or sulfur-containing chemical was used during the completion of the wells, or a formation water contains sulfide ion, zinc sulfide (ZnS) or iron sulfide (FeS, Fe2S3, FeS2) scales can form when a zinc bromide brine is commingled with sulfide ion and where soluble iron is available as a result of corrosion. The formation of zinc sulfide or iron sulfide scales can damage well productivity through a variety of associated problems such as 1) plugging of flow channels in the formation and across the perforation and 2) scaling on downhole tool assembles and surface facilities, etc.
To remove zinc sulfide or iron sulfide scales, an acid treatment is normally performed. After the acid treatment, the production rate may be restored to its previous level. However, new zinc sulfide or iron sulfide scale deposits can be formed in the well in a short period of time, and a re-treatment is required. This translates to increased costs. In addition, there are significant risks associated with acid treatments in high temperature, high-pressure gas wells. These include corrosivity of acid at high temperature and the generation of toxic H2S gas during the treatment.
Prevention of formation of zinc sulfide or iron sulfide scales may be effectuated by treating the zinc bromide brine with a zinc sulfide and iron sulfide scale inhibitor during well completion before scaling can occur downhole. There are several sulfide scale inhibitors on the market. These conventional sulfide scale inhibitors were originally developed for prevention of sulfate and carbonate scales in fresh water and low-density brines, such as KCl, NaCl or NH4Cl. These can be categorized into several families of chemicals such as phosphate esters, phosphonate/phosphonic acids, chelating agents, and polymeric inhibitors. Phosphonate/phosphonic acid type scale inhibitors have demonstrated their effectiveness to control ZnS scale at relatively low concentration. Polymeric scale inhibitors, such as sodium salt of acrylamido-methyl propane sulfonate/acrylic acid copolymer (AMPS/AA), phosphinated maleic copolymer (PHOS/MA) or sodium salt of polymaleic acid/acrylic acid/acrylamido-methyl propane sulfonate terpolymers (PMA/AMPS), are also effective scale inhibitors to control ZnS scales in fresh water and low density brines but not high density brines.
Although chelating agents have been reported to be effective scale inhibitors, their effectiveness is limited in heavy brines since their loading is in direct proportion to Zn2+ concentration. With chelating agents, stoichiometric reactions are typically required in order to form a stable complex between chelating agent and zinc ion. The high Zn2+ ion concentration of zinc bromide completion fluid makes this approach not practicable.
Unlike traditional scale inhibitors, especially those applied in fresh water or very low salt solutions (approximately 3 percent salt), inhibitors designed for zinc brines are generally required to be compatible with more than 50 percent dissolved salt. Thus, in zinc brines, the extremely high salt content and the limited amount of free water severely restricts both the solubility and effectiveness of the inhibitor. Thus, it is not practical to use a chelating agent in a completion brine that contains significant amount of Zn2+ ions.
For the majority of the scale inhibitors mentioned above, especially for polymeric inhibitors, their solubility in high density completion brines is very limited due to the high concentration of Zn2+ and Ca2+ ions present in the brines and very limited free water available. Further, high concentrations of Zn2+ and Ca2+ ions are detrimental to the effectiveness of phosphonic based scale inhibitors.
Furthermore, due to very limited free water in high-density brines, it is very difficult for a scale inhibitor to completely dissolve in the brine. During well completion operations, the completion brine is under constant circulation and filtration. Any scale inhibitor that is not totally soluble in the brine can be removed from the system and lose its effectiveness during the operation.
A need exists therefore for new zinc sulfide or iron sulfide scale inhibitors that have good solubility in high-density brines and which can effectively control ZnS or iron sulfide scaling.